Hello, I am Keith, one of the experts on Just Answer, and pleased to be able to help you with your question. Just Answer is a tax advice service and has no connection with HMRC.
This underpaid tax is due by 31 January 2017. You should check that the figures quoted by HMRC are correct. If they are not then you should appeal against the assessment which you do merely by writing a letter to the tax office.
Normally your 16/17 tax code should be 1100. However HMRC may be using the coding to recover underpaid tax which they can do for a debt under 3K. Your notice of coding should explain the reason.
I do hope that ,my reply has been of some assistance.